Dislocation

• Usually caused by sudden flexion and external rotation of tibia on femur, with simultaneous contraction of quadriceps muscles (eg, twisting motion, sudden lateral cut, or a direct impact)
• Lateral dislocation of the patella is most common, with the patella displaced over the lateral femoral condyle
• Uncommon dislocations include superior, medial, and intra-articular dislocation
• Direct trauma to patella

Fracture

• Direct trauma:
  • Most common mechanism
  • Secondary to direct blow or fall on patella
  • Usually results in comminuted or minimally displaced fracture, or open injury
• Indirect forces:
  • Avulsion injury from sudden contraction of the quadriceps tendon
  • Usually results in transverse or displaced fracture (often both)
• Types of patellar fractures:
  • Transverse: 50–80% (usually middle or lower third of patella)
  • Comminuted (or stellate): 30–35%
  • Longitudinal: 25%
  • Osteochondral

Patellar Tendon Rupture

• Usually caused by forceful eccentric contraction of quadriceps muscle on a flexed knee during deceleration (eg, jump landing and weight lifting)
• Often occurs in middle-aged athletes

Patellar Tendinitis

Overuse syndrome from repeated acceleration and deceleration (jumping, landing)

Dislocation

• Risk factors for patellar dislocation:
  • Genu valgum (knock-knee)
  • Genu recurvatum (hyperextension of knee)
  • Shallow lateral femoral condyle
  • Deficient vastus medialis
  • Lateral insertion of patellar tendon
  • Shallow patellar groove
  • Patella alta (high-riding patella)
  • Deformed patella
  • Pes planus (flatfoot)
• Common injury in adolescent athletes, especially girls
• Patellar dislocations most commonly occur in individuals aged 25 years or younger, with the highest incidence in the 10- to 17-year-old age group
• The younger the patient at the time of initial dislocation, the greater the risk of recurrence

Fracture

• Direct trauma
• Indirect forces caused by forcible quadriceps tendon contraction
• Male:female ratio 2:1
• Highest incidence in those 20–50 yr old

Patellar Tendon Rupture

• Peak incidence in 3rd and 4th decades:
  • Often in athletically active individual
  • Most commonly seen in men
• Risk factors:
  • History of patellar tendinitis
  • History of diabetes mellitus, previous steroid injections, rheumatoid arthritis, gout, systemic lupus erythematosus
  • Previous major knee surgery

Patellar Tendinitis

• Microtears of tendon matrix from overuse
• Seen in high jumpers, volleyball and basketball players, runners
• Seen commonly in younger patients, and those with a greater BMI